Not all warm-ups are the same. The days of static stretches in a circle are gone. Dynamic warm-ups, getting the heart rate up, and warming the body temperature up, first...has been proven to be more effective.
Not all kids need to hit 30 balls to get right with their swing before a game. Some only need a few balls while others only need dry swings. It is not a one size fits all world. Warming up is just like everything else...it's different for everybody.
Lastly, do not forget to do a proper cool down. Think about it, if you are able to make it through a sporting event injury free, consider yourself lucky. Don't push your luck by increasing the soreness over the next few days because you didn't allow the body to properly recover.
HYDRATE HYDRATE HYDRATE